I think a lot of people who say they have 18 inch arms carry about 2 inches of fat so if they were lean they would really only have sixteen inch arms. I was only about 25 lbs overwieght at one point and when I lost the wieght my arms lost an inch even though they actually have more muscle now.

Few people actually realize the effect that even "small amounts" of body fat have on measurements (which is relative to what you consider "small amounts"). At 15% body fat my arms are about 1/8" less than what the formula predicts, but the last time I went down to 8% my arms were a full inch off the prediction (though I didn't directly train arms while dieting which probably cost me a fair bit of muscle off my arms). I think that if I leaned out while training to maintain arm mass I might come within 1/2" of the prediction.

Incidentally, my lean body mass at 8% is about 5-6 pounds less than the formula predicts as my maximum (and I've been training very seriously for over 15 years). For a person of my height and structure a balanced gain of 5-6 pounds of muscle equates to about a 1/2" on the upper arms, which would put both my arm measurement and my body weight roughly where the formula says they "should be".

Realistically though, I'll probably never get that additional 5-6 pounds of lean body mass -- in my 15 years of involvement with bodybuilding I've seen only a few people who legitimately reached the formulae's predictions in lean condition without drug use ...and they were all high-level, drug-tested competitors. Most current high-level "natural" competitors hover somewhere around or slightly under the formulae's predictions for most body parts (in fact, before the most recent edition of that article was posted it was read by several world-class drug-free bodybuilders, none of whom claimed to exceed the predictions).